Can't seem to control out point properly

I'm new to motion and have watched a fair few tutorials but can't seem to control my out points.

When I type 600 into the right hand time box to make my sequence 6 seconds long and then drag out my (in this case)text bar out to 6 seconds, I then try and drag my out point to 6 seconds but it stops at 5:23, this seems to be the same for any whole number i.e. 4 seconds and my outpoint won't move past 3:23.